Ultra-fine microfabrication method using an energy beam

ABSTRACT

An ultra-fine microfabrication method using an energy beam is based on the use of shielding provided by nanometer or micrometer sized micro-particles to produce a variety of three-dimensional fine structures which have not been possible by the traditional photolithographic technique which is basically designed to produce two-dimensional structures. When the basis technique of radiation of an energy beam and shielding is combined with a shield positioning technique using a magnetic, electrical field or laser beam, with or without the additional chemical effects provided by reactive gas particle beams or solutions, fine structures of very high aspect ratios can be produced with precision. Applications of devices having the fine structures produced by the method include wavelength shifting in optical communications, quantum effect devices and intensive laser devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a method of fabricatingmaterials, and relates in particular to a method of ultra-finemicrofabrication using an energy beam to fabricate next generation VLSIdevices, ultra-fine structures, quantum effect devices andmicro-machined devices, and relates also to evaluating the fabricationproperties of the energy beam using the method.

2. Description of the Related Art

Photolithography and photomasking to generate a device pattern on asubstrate base have been an essential part of fabrication ofsemiconductor devices. A photolithographic device fabrication process isbased on masking those regions of the substrate base which are not to beetched with a photoresist mask, and etching the base material away fromthose regions which are not protected by a photoresist mask, therebyproducing on the fabrication surface ditches or recesses whose depthsare dependent on the duration of etching.

FIGS. 15A-15E illustrate the processing steps (step 1 through step 5,respectively) involved in the conventional technique based on the use ofphotoresist masking. In step 1, the surface of a substrate base 1 iscoated with a photoresist material 2. In step 2, ultraviolet light 4 isradiated on the photoresist material 2 through a photomask 3 placed ontop of the coated base 1, thereby transferring device patterns 3a formedin photomask 3 to the photoresist material 2. In step 3, the photoresistmaterial 2 exposed by the photomask 3 is removed in a photographicdevelopment process to leave behind only the unexposed regions of thephotoresist material on the base 1. In the following step 4,unisotropical etching is carried out to remove the base material fromthe fabrication surface by using ions or radicals in a plasma etchingprocess on those bare regions of the base 1 not protected by thephotoresist material 2. In the final step 5, the photoresist material 2is removed. These five steps are essential in the conventional techniqueto duplicate the pattern 3a of the photomask 3 by using photolithographyto form ultra-fine ditches or recesses 1c in the surface of the base 1.In general, it is necessary to repeat those basic five steps a number oftimes to form ditches of different depths in the base 1 before anoperative semiconductor device can be produced.

Therefore, throughout the process of conventional microfabricationpresented above, various photomasks 3 having different complexphotoresist patterns 3a are absolutely essential, and furthermore, linesor holes in the range of 1 μm or less are required in the photomasks,special equipment and effort are required, and both capital and laborexpenses associated with the technique are rather high. Even with thebest of equipment, the technique is basically not adaptable tomicrofabrication in the range of nanometers. Also, for the technique tobe practical, photoresist material 2 must respond to ultraviolet lightor electron beams, thereby limiting the choice of photoresist materialwhich can be used. Further, the use of the technique is not allowed whenthere is a danger of the photoresist material becoming a contaminant.Further, the success of photolithography is predicated on preciseflatness of the surface of the substrate base so that the entirefabrication surface lies on a flat plane, to enable uniform fabricationof the entire surface of the substrate base. When the fabricationsurface lacks flatness or smoothness, it is not possible to produce aphotoresist film of high uniformity and to produce a precise exposureover the entire surface.

Further, in using the conventional plasma etching process to producepatterns of less than 1 Am in size, because of the collision among thegas particles and charge accumulation on the resist material, too manyof the charged particles are deviated from linearity, and strike thesurface at some non-perpendicular angles to the surface. Under suchconditions, it is difficult to produce deep vertical ditches or recesseshaving a high aspect ratio (a ratio of depth to width), and furthermore,it is nearly impossible to manufacture three-dimensional structuralpatterns having a width of less than 1 μm.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of energybeam-assisted ultra-fine microfabrication to enable fabrication of finestructures in a nanometer range by dispersing micro-particles as beamshielding means on a fabrication surface of a target object, andradiating the surface with an energy beam to produce the finestructures. The dispersion patterns of the microparticles can becontrolled as necessary by applying a magnetic or electric field or alaser beam. To produce uniformly etched fine structures or finestructures having high aspect ratios, the etching process by energy beamradiation is followed by etching with chemically reactive gaseousparticles or in a chemical solution. The method enables the productionof ultra-fine structures which are dimensionally precise and have a highaspect ratio.

Another object of the present invention is to provide a method ofevaluating the fabrication properties of an energy beam for use inmicrofabrication, such as fabricating speed, area, depth as well asunisotropical etching properties and other fabrication parameters suchas the condition of the surface produced by the energy beam.

The first object is achieved in a method comprising the steps ofdispersing and positioning micro-particles having particle sizes inranges of one of from 1-10 nm, 2 from 10-100 nm and 2 from 100 nm to 10μm, for shielding regions of a fabrication surface of a target objectform exposure to an energy beam, and radiating the energy beam on thefabrication surface so as to produce a fine structure by an etchingaction of the energy beam on those regions of the fabrication surfacewhich are not shielded.

An aspect of the method is that while the energy beam is fabricating thefabrication surface in the depth direction, the shielding member is alsoetched away to produce a cone-shaped fine structure. To enable themicrofabrication method, dispersion of the micro-particles can beperformed in one of two ways, i.e., dripping a solution containing themicro-particles onto the fabrication surface and drying themicro-particles on the surface, or immersing the target object in thesolution and drying the micro-particles on the surface.

Another aspect of the method is that positioning of the shieldingmembers is performed with the use of a laser beam or a magnetic/electricfield on the micro-particles, and the energy is beam-fabricated objectis further processed with a reactive gas, under thermal control of thefabrication surface, to produce an isotropically etched fine structure.

The method as summarized above, based on shielding provided by nano-sizeor micron-size particles and etching with an energy beam, makes itpossible to produce fine structures which were not possible within thescope of conventional fabrication methods based on photolithographictechniques.

The object of providing an evaluation method of an energy beam isachieved in a method comprising the steps of attaching microparticleshaving a specific range of particle sizes on a fabrication surface of atarget object, radiating an energy beam onto the fabrication surface fora specific time interval for producing the fine structure, and analyzingstructural features of the fine structure.

When evaluating the fabrication properties of an energy beam, the sizeof the fine pattern is an important parameter. In the method of thepresent invention, the fine patterns are produced by placing andattaching micro-particles in any desired patterns on the test surfacewithout being restricted by the availability of pre-fabricated patterns.Therefore, fine patterns of a wide range of sizes can be used withvarious energy beams to evaluate the fabricated depth and shapes byobserving the fabricated surface with an electron microscope, therebypermitting evaluation of a wide variety of energy beams. The method isalso applicable to a wide variety of qualities of the surface to befabricated, which presented problems in the conventionalphotolithography technology, because the micro-particles may be simplydispersed on and attached in situ to the fabrication surface withoutbeing restricted by surface roughness or lack of flatness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a target object having microparticlesattached to a fabrication surface.

FIGS. 2A-2C illustrate processing steps in a first method ofmicrofabrication of the present invention.

FIGS. 3A-3E illustrate processing steps in a second method ofmicrofabrication of the present invention.

FIG. 4 is a perspective view of an example of fabrication performed byarranging micro-particles in a two-dimensional periodic pattern on afabrication surface of a target object, performed according to themicrofabrication method of the present invention.

FIG. 5 is a perspective view of another example of fabrication performedby arranging micro-particles in a two-dimensional periodic pattern on afabrication surface a target object, performed according to themicrofabrication method of the present invention.

FIG. 6 is a cross-sectional view of an example of fabrication of twotip-shaped fine structures produces by the microfabrication method ofthe invention, each having a specific arrangement of rod-shaped finestructures, on mutually intersecting planes.

FIG. 7 is a side view of an example of a cone-shaped structure of Sifabricated on a target object according to a microfabrication method ofthe present invention.

FIG. 8 is a perspective view of three-dimensional fine structuresfabricated on a plurality of faces of a target object, producedaccording to the microfabrication method of the present invention.

FIG. 9A is an enlarged cross-sectional view of a spot on a targetsurface to illustrate the effect of distance form a center C of anenergy beam on the depth d of etching achieved.

FIG. 9B is a graph showing a curve P relating the distance x from thecenter C of an energy beam on the x-axis and the fabrication depth d onthe y-axis.

FIG. 10 is a schematic illustration of an apparatus for evaluating thefocusing property of a fast atomic beam.

FIGS. 11A-11C are graphs showing the effects of changing a parameter Lin the apparatus shown in FIG. 10 on the depth d of etching.

FIGS. 12A-12C are perspective views of objects produced by themicrofabrication method by placing rod-shaped shielding members in aperiodic pattern.

FIGS. 13A and 13B are schematic views of objects produced by placing aphotoresist film in a periodic pattern.

FIG. 14 is a schematic illustration of an example of a target objectfabricated by a microfabrication method by moving a target object.

FIGS. 15A-15E illustrate five basic processing steps required in theconventional photolithography method of microfabrication.

PREFERRED EMBODIMENTS OF THE INVENTION

In the following, preferred embodiments will be explained with referenceto FIGS. 1 to 14.

FIG. 1 is a perspective view of micro-particles 12 attached to afabrication surface of a target object 11. The drawing illustrates anumber of particles, wherein the size of the particle is of apredetermined size in a range on the order of nm or μm. The targetobject may be any of semiconductor materials including at least one ofGaAs, Si, SiO₂, insulation materials including at least one of glass andceramics and conductive materials such as metals.

A method of dispersing and attaching the micro-particles 12 on thefabrication surface of the target object 11 is to disperse themicro-particles 12 in a solvent such as ethanol and the like, stirredwith a surfactant to obtain uniform dispersion. The target object 11 canthen be immersed in the solution, or the solution can be dripped ontothe fabrication surface to cover the fabrication surface with thedispersion solution. Subsequently, the solvent is removed by evaporationto leave only the micro-particles 12 in situ and uniformly distributedon the fabrication surface.

The particle diameter of the micro-particles 12 may be in the range of:one of 0.1-10 nm; 10-100 nm; and 100 nm to 10 μm. Ferrite, zinc, cobaltand diamond particles are in the 0.1-10 nm range, and aluminum,graphite, gold an silver particles are in the range of 100 nm to 10 μm.It is preferable that the microparticles 12 be spherical but othershapes are also employable, in which case, the range of particle sizesmay be expressed in terms of the average diameter or the maximumdiameter. It is desirable that the particle size be uniform, but this isnot an essential requirement. If the size distribution is wide, apreferred range of particle sizes should be selected.

FIGS. 2A-2C illustrate processing steps in a first method of ultra-finemicrofabrication method using an energy beam (shortened tomicrofabrication method hereinbelow).

As shown in FIG. 2A, micro-particles 12 made of materials such ascobalt, zinc and ferrite, having diameters ranging from 5 nm to 1 μm,are distributed on the surface of a target object 11 made of GaAs, Si orglass. The micro-particles 12, distributed and attached to thefabrication surface by one of the two methods discussed above to shieldthe fabrication surface from the energy beam, are distributed uniformlywith statistical accuracy.

Next, a fast atomic beam (FAB) 13 as an energy beam is radiatedapproximately perpendicular to the target object 11 along the directionshown by the arrow in FIG. 2A. Because the regions protected by themicro-particles 12 are shielded from the FAB 13, only those regionswhich are not protected by the micro-particles 12 are etched by the FAB13, and the etching process proceeds as illustrated in FIG. 2B.

In this case, etching by the FAB 13 is effected not only on the targetobject 11 but also on the micro-particles 12 which gradually becomesmaller with continuing exposure to the radiation. However, the speed ofshrinking of the micro-particles 12 will depend on their reactivity withthe gas used for the FAB 13. To produce a fine-structure having a highaspect ratio, therefor, the FAB 13 is chosen to be either a rare gas ora gas having a low reactivity with the micro-particles 12 but a highreactivity with the material of the target object 11 so that shrinkingof the micro-particles 12 can be controlled to be as little as possible.The result is that it is possible to leave only those shielded areas ofthe target object 11 below the micro-particles 12 unetched to producevertical walls of a rod-shaped fine structure. Thus the structureremaining on the surface of the target object 11 is a fine structure ofrod-shaped protrusions whose diameter ranges between 5 nm to 1 μm. Theuniformity of the structure is dependent on the uniformity of theparticle size of the micro-particles 12.

After the radiation processing of the target object 11 with an energybeam has been completed, gaseous particles of chlorine or fluorine forexample, which are reactive with the target object 11, may be introducedwhile heating the target object 11 with a heater or an infrared lamp.Then, isotropical etching can be performed as illustrated in FIG. 2C. Byusing such an isotropical etching process, it is possible to etch boththe micro-particles 12 and the rod-shaped fine structure 14 made of thematerial of the target object 11. Compared with the structure 14 shownin FIG. 2B, produced with FAB 13 only, it can be seen that a finerrod-shaped structure 14a can be produced. The residual micro-particles12 which remain after the final etching process are usually unwanted,and they may be removed from the top of the rod-shaped structure 14a bya material removal process such as water-jet cleaning.

The type of the target object material, which can be processed by theenergy-beam assisted microfabrication method, is not particularlyrestricted, and includes semiconductor substrate materials such as atleast one of GaAs, Si SiO₂, insulation materials such as at least one ofglass and ceramics, as well as metallic materials. For micro-particles12, if ultra-fine particles of a particle size below 0.1 μm arerequired, at least one of ferrite, zinc, cobalt and diamond may be used,or if the particle size required is between 0.1 to 10 μm,micro-particles 12 of at least one of aluminum, graphite, gold andsolver may be used. The selection of the material for micro-particles 12should be made on the basis of its reactivity with the etching gas orits sputtering property.

FIGS. 3A-3E illustrate the processing steps in a second method of themicrofabrication method. In the first embodiment the energy beam usedwas a fast atomic beam 13 of a rare gas or a gas which has a highreactivity with the base material but a low reactivity with themicro-particles to suppress the etching action on the micro-particles12. FIGS. 3A-3E present a slightly different aspect of themicrofabrication method. In this case, some allowance is made for theshape change (shrinking) in the micro-particles, and the target object21 in this case is one of III-V group semiconductor materials, such asone of GaAs, AlGaAs, InAs. Fast atomic beam (FAB) 23 is gaseouschlorine, and the micro-particles 22 are nano-particles of diamond of aparticle size between 1 to 50 nm which are reactive with the FAB 23.

First, as shown in FIG. 3A, diamond nano-particles 22 are dispersed onthe surface of target object 21, and FAB 23 of gaseous chlorine isdirected as shown by the arrow in FIG. 3A. The process of etchingproceeds on the fabrication surface of the target object 21, asillustrated in FIGS. 3B, 3C, but at the same time, the diamond particles22 are also etched, albeit at a low speed. As the size of the diamondparticles shrinks, the protective shielding provided by the diamondparticles 22 against the FAB 23 is also reduced. As the fabricationprocess is continued, that portion of the target object 21 which isprotected by the diamond particle 22 decreases in size to produce afine-tipped structure 24 as illustrated in FIG. 3D. When the process iscontinued until the diamond particle 22 disappears, the final structureproduced is a cone-shaped fine structure 24 as shown in FIG. 3E. As anexample, a cone-shaped fine structure 24 produced during testingexhibited a respectably high aspect ratio consisting of a tip diameterof 10 nm and a height of about 250 nm.

After the completion of the energy beam radiation step, furtherrefinement on the tip shape of the fine-tipped structure 24 may becarried out by using isotropical etching by introducing only gaseouschlorine and heating the target object 21 with a heater or an infraredlamp. In such a case, a fine structure 24 having a tip dimension of 0.1to 5.0 nm may be produced. By thermally assisting the chemical reactionin the post-FAB stage, it is possible to smooth out the surface orremove damaged surface layers from the surface of the target object 21.This approach is particularly effective in the production of quantumeffect devices. Quantum effect refers to a property associated with afine structure of a bulk material which is different than thecorresponding property in the body or base of the bulk material itself,for example, shifting of the wavelength of input light to a shorterwavelength or a shift in the electron energy level. Using the quantumeffect, it is possible to shift the wavelength of an output light beamor laser beam emitted from the fine structure to a shorter wavelengthcompared to the bulk wavelength. When a laser beam is passed through thefine structure, the wavelength of the output laser beam from the tipshifts to a shorter wavelength compared to the input wavelength, therebyincreasing the output power of the laser emitted from the finestructure.

FIGS. 4 and 5 show examples of fabrication to produce fine structures30, 40 by arranging the micro-particles in a two-dimensional periodicpattern on the fabrication surface of target object materials 31, 41.The fine structure 30 shown in FIG. 4 comprises an array of fine columns34 (i.e., rod-shaped structures) on the surface of the target object 31according to a given pattern (i.e., matrix distribution pattern). thefine structure 40 shown in FIG. 5 comprises an array of fine cones 44(i.e., rod-shaped structures) on the surface of the target object 41according to a matrix distribution pattern. The method of fabrication isbasically the same as those described above, however, the control of thedistribution of micro-particles 32 is based on a further advancement inthe microfabrication method, carried out with laser-assisted,electric-field assisted, or magnetic-field assisted distribution ofmicro-particles, to control the matrix distribution pattern according toperiodic distribution rather than statistical distribution. The effectof laser radiation is to ionize the micro-particles 32 themselves orionize the boundaries of the micro-particles 32 to generate plasmastatus. Therefore, those micro-particles 32 which are radiated with alaser beam become ionized and charged. By applying an electric field ona laser-focused spot, the micro-particles 32 can be captured (trapped)and moved to a desired location. The resulting distribution is themicro-particles 32 being disposed on the target objects 31, 41 accordingto a matrix distribution pattern as illustrated in FIGS. 4 and 5.

When using an electric field effect instead of a laser beam, an electricfield is applied to trapping electrodes to trap the micro-particles 32and move them to the desired locations to produce a periodic array offine structures. Further, when using a magnetic field effect, it ispossible to utilize magnetic micro-particles such as ferrite powdersdispersed in a solution, and produce a matrix distribution pattern alongthe magnetic flux lines of a magnetic field produced by electromagnetsor permanent magnets. When the micro-particles 32 are to be distributedon a scale of nanometers, it is possible to utilize a piezo-electricelement and apply a controlled voltage through electrodes or magneticpoles to expand or contract the piezo-electric element in a nanometerrange to produce a desired matrix distribution pattern of desiredaccuracy.

FIG. 6 is an example of the microfabrication method applied to a targetobject 50 having a periodic distribution pattern of a line or row ofrod-shaped fine structures 54 disposed on each of two mutuallyintersecting side planes 51a, 51b of a target object 51 oriented at someangle. The diameter of the fine structure 54 is between 1 to 20 nm, andthe height is between 10 to 500 nm. An optical axis passing through eachof the two lines of the fine structure 54 merges at an intersectionpoint P, and each of the fine structures 54 functions as a quantumeffect optical amplifier to the laser beam input from each of lasergenerators 55. Such a device having the fine structures 54 can alsofunction as a quantum effect laser generator when mirror resonators areplaced on both sides of the fine structures 54. In such an opticaldevice, the wavelength of the output laser can be shifted to a shorterwavelength or the device may act as an intense power optical oscillatorso that an optical pulse field of extremely high power may be generatedat the intersection point P. It will be recognized by those skilled inthe art that such an intense power laser beam from such a device wouldhave applications in many scientific and technical fields, includingirradiation of materials to generate light, sputtering of atoms,ionization and severing of atomic chains, in addition to trapping ofmicro-particles developed in the methods described above.

The fabrication target material 60 shown in FIG. 7 is made on a siliconsubstrate base 61 in accordance with the steps presented in the secondembodiment described above. Because the base material is silicon, theenergy beam utilized is a fast atomic beam based on gaseous particlesfrom a fluorine group, such as SF₆. Isotropical fabrication is performedin a post-FAB processing step, by producing a plasma of fluorine-groupgas particles, and supplying a high concentration of fluorine radicalsto the fabrication surface. The cone-shaped Si fine structure 64 thusproduced can be used as a cantilever needle point in an atomic fieldmicroscope (AFM) or scanning tunnelling microscope (STM). The needle tipis used to examine the surface roughness configuration of a targetobject, based on the vertical movement of the needle tip moving alongthe surface. Because of the sharpness of the needle tip of the finestructure 64, it can make concentration of an electric field easy, andcan, therefore, be used as an electron emission source in field emissionapplications. Field emission is a technique of generating a controlledbeam of electrons by emitting an electron beam from a micro-emitter(such as the needle tip fine structure illustrated) encased in aninsulator tube, and regulating the voltage on beam-guiding electrodesdisposed at a beam exit port of the insulator tube to control theemission process. This type of electron beam can be used in electronbeam drawing devices for nanometer range drawing.

FIG. 8 shows an example of a three-dimensional multi-surfaced element 70produced by the microfabrication method of the invention. Thecone-shaped fine structures 74 and the rod-shaped fine structures 75 areproduced, according to the methods described above, on a top surface 71aand a side surface 71b, respectively, of a three-dimensional targetobject 71. Each of these fine structures 74, 75 exhibits a differentquantum effect. The cone-shaped fine structures 74 having resonatormirrors 74a, 74b act as optical amplifiers for a laser beam L1, and therod-shaped fine structures 75 having resonator mirrors 75a, 75b act asoptical amplifiers for a laser beam L2. The fine structures 74, 75generate laser beams of different wavelengths according to differencesin their quantum effects.

In more detail, the generated laser beam L1 output through the outputmirror 74b and the generated laser beam L2 output through the outputmirror 75b have different wavelengths, but each beam is directed to arespective reflection mirror 76, 77 to change the direction ofpropagation at right angles, and is then directed to front face 71c ofthe target object 71. The laser beams guided to the front face 71c areseparated or coupled by rotation mirror 78, and are input into aphotodetector 79. The rotation mirror 78 is an optical mirror having adifferent wavelength selectivity on its front surface than its backsurface so that one wavelength may be filtered or coupled to the otherwavelength depending on the phase relation at the rotation mirror 78,thereby filtering or mixing the wavelengths of the laser beams L1 andL2.

The element 70, having the fine structural features presented above, canalso be utilized in the field of optical communications. In this case,when a data group comprised of two different wavelengths is propagatedthrough a common bus line or separate bus lines, the transmission timeis halved compared with the case of propagating 16-bit data on a singlewavelength. This is because the pulse data having two differentwavelengths can be transmitted simultaneously through an optical fiberas a common bus line. In such an application, the photodetector 79 isprovided with wavelength selectors for two wavelengths λ1, λ2, eachhaving eight data lines for outputting data through preamplifiers andparallel signal processors to transmit 8-bit data to memories and otherperipheral devices. In other words, the element 70 is able tosimultaneously process 8-bit data carried on two wavelengths, thusenabling to transmit 2×8=16 bits of data in half the time. Similarly, byincreasing the number of wavelengths, a multiple integer-number increasein data transmission speed can be achieved in the same duration of time.By having a plurality of elements 70, a large number of groups ofinformation can be transmitted, depending on the number of wavelengthschosen, thus realizing a high capacity optical data transmission device.

Instead of the rotation mirror 78, a half mirror or anelectrically-controlled polarizing mirror may also be used. Thephotodetector 79 may be replaced with two optical sensors tuned to twowavelengths λ1, λ2, and although it would be necessary to match thephases in this case, there would be no need for the output selectionmirror 78. Another possibility is to provide more than two rod-shaped orcone-shaped fine structures on the element 70 so that many data groupscan be transmitted on many different wavelengths.

A summary of the overall salient features of the energy beam-assistedmicrofabrication method will be reviewed below.

The method is based on attaching an positioning of shielding members,including micro-particles, on the surface of a target object, with arange of particle sizes of one of 1-10 nm, 10-100 nm and 100 nm to 10μm. when an energy beam is radiated onto the surface, those areas notshielded ar fabricated, i.e., etched, thereby producing fine structures,which are of the order of the size of or smaller than the shieldingmembers, that cannot be fabricated by the conventional photolithographyapproach. Because the shielding members are placed on the surface to befabricated, they are free from rigid requirements of roughness orflatness of the surface. This permits a three-dimensional fine structureto be fabricated by placing the shielding members at any local areas ordifferent faces of a target object. In contrast, the conventionalphotolithography is limited to highly flat, smooth two-dimensionalsurfaces. Otherwise, a photoresist film of a quality acceptable forphotolithography cannot be produced on the surface. By combining thetechnique of unisotropical etching with the superior linearity of anenergy beam, flexibility in fabrication can be further enhanced. Bycombining the method with the technique of positioning of themicro-particles on the surface by the use of a laser ormagnetic-electrical field, it is possible to fabricate ultra-finestructures applicable to quantum effect devices, electronic and opticaldevices which are important not only in academic studies, but inindustrial products of the future.

Another feature of the method of the invention is that it is able toproduce fine-tipped structures on any type of material by a judiciouscombination of energy beams, material for the shielding member and thetarget material. When the etching condition is chosen to benon-selective, for example if the energy beam is based on gas particlesreactive to the shielding member as well as to the target material, afine-tipped structure can be produced by etching away the shieldingmember as well as the target material so that a cone-shaped structure isproduced. On the other hand, when the etching condition is chosen to beselective, for example if the energy beam is based on gas particlesreactive to the target material but not to the shielding member,shrinking of the micro-particles is suppressed, and a rod-shaped finestructure having vertical walls can be produced.

Shape control of the fine structures can be exercised in other ways. Forexample, if heat is applied locally to a fine structure to thermallycontrol its reaction rate, it is possible to isotropically etch the finestructure. After fabricating a target object with an energy beam, areactive particle of a gas, such as chlorine or fluorine gas, isintroduced while heating the fine structure with a localized heatingdevice so as to etch the fine structure to provide an isotropicallyetched structure. This approach produces etching not only on themicro-particles but also on the fine structure so that, compared withthe case of fabrication by FAB only, it is possible to produce finerstructures having controlled cross-sectional shapes.

A further advantage of the method of the invention is that an energybeam can be chosen from a wide variety of energy beams including one ofa fast atomic beam, an ion or electron beam, a laser or radiation beam,and an atomic or molecular beam, depending on the nature of the targetmaterial and the micro-particles as the shielding member. For example,an electrically neutral fast atomic beam may be applied to one ofmetals, semiconductor and insulation materials as well as to many othertypes of materials. An ion beam is particularly effective for metals,and an electron beam with a reactive gas particle beam is useful ineffecting fine localized etching of only the area radiated by theelectron beam. A radiation beam is useful when used alone or withreactive gas particles to provide chemically-assisted fabrication basedon a mutual interaction of the gas with the target material. An atomicor molecular beam based on reactive gas particles is useful to provide alow energy beam fabrication method.

Next, a method will be presented of evaluating the fabricationproperties of an energy beam such as a fast atomic beam (FAB) forproducing fine structures by radiating through a shielding patternarranged on a fabrication surface of a target object. Some of theseproperties are operating parameters such as fabrication speed, area, anddepth as well as properties related to the nature of the beam, such asselectivity and shaping capabilities.

FIG. 9A shows the depth of fabrication produced by a focused energy beam13 as a function of the distance x from the center C of the beam 13.FIG. 9B shows a curve P relating the depths plotted on the y-axis to theposition along the distance x plotted on the x-axis. As shown in FIGS.9A, 9B, it is possible to evaluate the performance of an energy beam bydispersing the micro-particles 12 on the surface of a target object,radiating the surface perpendicularly with a focused beam to fabricatethe surface, and observing the shape and the depth d of the finestructures 14 produced. The results are indicative of the distributionof local energies of the energy beam 13, of the depths of fabrication,of the density of the energy beam, and are also indicative offabrication properties such as selectivity and penetration capabilitiesof the energy beam 13.

FIG. 10 is a schematic diagram of an apparatus used for evaluating thefocusing performance of an FAB. The FAB generation device, regarded asan efficient means of microfabrication, was redesigned in thisinvention. Discharge electrodes and discharge ports were designed sothat a plurality of high speed particles 25 emitted from a plurality ofbeam discharge ports of the FAB source 18 can be focused at a focalpoint F. The focal point F was chosen to that it was located at adistance L (in mm) below fabrication surface 11 a of target object 11which is GaAs, and the fabrication surface 11a of the target 11 washeated with an infrared lamp 35. In this experiment, micro-particleshaving a particle size of several μm were attached to the surface 11a ata distribution density of several particles per 100 μm².

FIGS. 11A-11C are graphs showing the dependence of the depth of etchingd (in μm) on the distance L (in mm) from the surface 11a to the focalpoint F. The distance L is positive (+) below the surface, and thehorizontal axis is the same as in FIG. 9B, and represents the distance x(in mm) from the center C of the energy beam. The depth measurementswere made with an electron microscope. FIGS. 11A-11C indicate that thedepth of etching becomes larger towards the center C of the beam (xbecomes closer to zero). The results indicate that the energy particledensity is higher towards the focal point F. It follows that when L=0, adeep hole can be produced at the center of the beam, and as the distanceL is increased (by moving the focal point F away from the surface 11a),etching becomes shallow but occurs over a wide area.

The salient features of the present invention presented to this pointnow will be briefly reviewed.

The findings demonstrated that etching of lines or spots of a diameterless than 1 μm (which are difficult to achieve with conventionaltechnology) can be achieved easily by dispersing and attachingmicro-particles of a given diameter on the fabrication surface of amaterial and processing with an energy beam. The micro-particles can beproduced fairly readily over a wide range of particle sizes employablefor the present invention. Thus, a relatively wide range of patternsizes can be produced by the method of the invention. The performancecharacteristics of the energy beam can be evaluated simply andaccurately, as described above, by dispersing and attachingmicro-particles and radiating the fabrication surface of a material witha suitable energy beam, and studying the topography of the etchedsurface by electron microscopy.

Tests have also confirmed that even when the surface roughness of thetarget object is not suitable for the conventional photolithographicapproach, the method of the present invention enables patterns to beproduced over a much wider range of sized and depths than are achievableby the conventional photolithographic method, thereby making it possibleto evaluate the performance characteristics of any energy beam over awide range of sizes and patterns.

Energy beam is a general term used to describe a variety of energeticbeams employable in the microfabrication method, such as a fast atomicbeam, an ion beam, an electron beam, a laser beam, a radiation beam, anatomic or a molecular beam. The characteristics of these various beamsare briefly summarized below. A fast atomic beam is a neutral particlebeam and is applicable to all types of metals, semiconductors orinsulators. An ion beam is particularly effective for metals. Anelectron beam is useful in localized processing when used in conjunctionwith reactive gases. A radiation beam is applicable to a variety offabrication requirements when used alone or with reactive gases toutilize mutual interaction of the gas with the target material. Anatomic or molecular beam is based on atomic particles or molecularparticles of reactive gases, and is effectively used for providing a lowenergy beam for low damage fabrication.

Next, some examples of fabricating an arrayed structure by themicrofabrication method of the invention will be demonstrated withreference to FIGS. 12A-12C through FIG. 14.

FIG. 12A shows a basic approach of the method. Rod-shaped shieldingmembers 80 (for example, of a width ranging from one of 0.1-10 nm,10-100 nm and 100 nm to 10 μm) are placed on the surface of a targetobject 11 (substrate base) to shield energy beam B directed from above.There is no need for holding devices for the shielding members 80, andit is sufficient to just position them in a suitable pattern. Bychoosing the type of energy beam B appropriately, the target object 11is etched as illustrated in FIG. 12B to form an arrayed structurecomprising a series of parallel line protrusions 81.

A suitable energy beam for this type of application is a fast atomicbeam comprising uncharged particles having a good directional property.For the target object 11, Si, GaAs have been used, but othersemiconductor materials, insulators such as glass, quartz, and metalsmay also be processed. The shielding member 80 may be made byelectrolytic polishing of one of tungsten, gold, silver, platinum andnickel into a fine-wire form (of about 50 82 m diameter). The shieldingmembers 80 may be held in place in a suitable manner on the substratebase.

In place of the material-removal process of etching presented thus far,it is possible to perform a material-addition process, such as formingof a deposition film as illustrated in FIG. 12C, by selecting the beamtype and the energy level suitably. Using an energy level in a range ofseveral to hundreds of eV, and gaseous sources such as methane forcarbon together with an aluminum- or titanium-containing gas, insulativeor conductive films can be deposited on the beam-radiated areas, therebyduplicating the arrangement of the pattern made by the shielding members80.

The examples shown in FIGS. 13A and 13B relate to microfabrication by afast atomic beam 93 radiating a target object 91 coated with aphotoresist film 92.

In the past, an ion or electron beam has been used for fabrication.However, because of the loss of linearity of a charged particle beam dueto interference by electrical charges, or charge accumulation in thecase of insulating material, it has been difficult to performfabrication in the range of nanometers. Other fabrication techniques,such as reactive gas-assisted, ion beam or electron beam fabrication,involving the reaction of activated gas particles with the surfacematerial, were also inadequate. In these techniques, the reactive gasparticles are isotropic in their etching behavior, and directionaletching was difficult, and high precision microfabrication could not beperformed. In any case, none of the existing techniques is sufficientlyadaptable to microfabricate local areas as well as a large area.

Because the conventional photolithographic technique is incapable ofproducing fine patterns in the nanometer range, the fine patterns on thephotoresist film 92 on the surface of the target object 91 in theseexamples are produced by the technique of nano-lithography using SEM(Scanning Transmission Electron Microscope) or STM (Scanning TunnelingMicroscope) or with the use of electron-beam holography. The targetobject 91 is made of a semiconductor material such as Si, SiO₂ or GaAshaving pre-fabricated patterns of the photoresist film 92.

To microfabricate the target object 91 having a patterned photoresistfilm 92, a FAB 93 of a relatively large diameter is radiated to thesurface as illustrated in FIG. 13A. The FAB 93 is generated from anargon gas FAB source 97, described in U.S. Pat. No. 5,216,241 forexample, and comprising a discharge space formed by two (or three)flat-plate type electrodes 95, 96 contained in a housing 94, andintroducing argon gas into the discharge space. The argon FAB source 97produces an argon plasma in the discharge space by impressing a highvoltage between both electrodes 95 and 96. The ionized gas particles areattracted to the negative electrode 96, where they collide with the gasmolecules and combine with electrons to transform into fast atomicparticles. The neutral particles are discharged from a discharge port inthe negative electrode to provide a neutral fast particle beam. Therelative position of the FAB source 97 and the target object 91 may befixed or moved in a plane parallel to the target object 91 to produce atwo-dimensional pattern on the fabrication surface. In this example, theposition is shown to be fixed for convenience. Also, in the actualfabrication process, argon gas (Ar) is replaced with a gas having a highreactivity with the target object 91.

Because the FAB 93 emitted from the FAB source 97 is an electricallyneutral beam, it is not affected by charge accumulation or electrical ormagnetic fields, and exhibits a superior linearity. Therefore, the FAB93 can be made to penetrate straight into ultra-fine holes or grooves,thereby enabling to fabricate even the bottom surfaces of deep grooveshaving a high aspect ratio. In the case of the embodiment illustrated,the width of the patterns fabricated on the photoresist film 92 rangedbetween 0.1-100 nm. The fabricated depth d, indicated in FIG. 13B,depends on the aspect ratio, but precision in fabricating the depth dranged between 0.1-100 nm.

The results show that, unlike the problematic techniques ofmicrofabrication based on an ion or electron beam, an advantage ofmicrofabrication based on an electrically neutral FAB beam is that thesuperior linearity of the FAB is fully utilized to produce finestructures having a high aspect ratio, because a neutral FAB is notsusceptible to local or irregular changes in electrical potential whichaffect the behavior of the ion or electron beam, the neutral FAB doesnot adversely affect the properties of a target semiconductor orinsulator material. Therefore, the applicability of the technique is notlimited by the nature of the target material.

FIG. 14 illustrates another embodiment of the present invention in whichan ultra-fine beam 93, in the range of 0.1-100 nm diameter from a FABsource 97, is radiated onto the fabrication surface of a target object91 while moving the target object 91 relative to the source 97 so as tofabricate an ultra-fine three-dimensional pattern on the fabricationsurface. In this case, the beam diameter is adjusted to produce anultra-fine beam by disposing a beam aperture control device 98 betweenthe FAB source 97 and the target object 91. The beam aperture controldevice 98 is provided with two or more shield plates 99 havingultra-fine pin holes 99a, and the linearity of the beam is enhanced bypassing the beam through at least two pin holes 99a. The ultra-fine pinholes 99a are produced by removing atoms from the shield plate 99 whileobserving the images under a scanning tunnelling microscope (STM), forexample. The FAB 93 emitted from the FAB source 97 is constricted by thebeam aperture control device 98 to a required beam diameter, and isintensely focused on a target fabrication spot.

To produce a relative motion of the target object 91 with respect to theFAB source 97, the target object 91, in this case, is placed on amicro-manipulator stage which permits rotation/parallel movement (notshown) according to some pattern. In the case of target object 91 havingthe shape shown in FIG. 14, the FAB is always radiated in the negativedirection along the Z-axis during the fabrication process. A channel 104extending from an edge towards the middle of the target object 91, seenin the front face of the target object 91, is produced by orienting faceA of target object 91 perpendicular to the Z-axis and moving the targetobject 91 in the negative direction along a plane parallel to theX-axis. When the channel 104 is extended to the middle of the targetobject 91, the target object 91 is moved in the negative directionparallel to the Y-axis to produce the channel 105 which extends toanother edge of the target object 91. After thus completing thefabrication of two orthogonally intersecting channels 104 and 105, thetarget object 91 is rotated 90 degrees about the X-axis so that face Bof target object 91 is oriented perpendicular to the Z-axis. Next, thetarget object 91 is moved in the negative direction parallel to theY-axis to produce a channel 106 which extends towards the middle of thetarget object 91. When the channel 106 is extended to the middle of thetarget object 91, the target object 91 is moved in the negativedirection parallel to the X-axis to produce a channel 107 which extendsto another edge of the target object 91, thus producing another twoorthogonally intersecting channels 106 and 107.

Accordingly, by moving the micro-manipulator to provide rotationcombined with linear moving motions in a pre-programmed pattern, athree-dimensional pattern is fabricated on multiple surfaces of thetarget object 91. As in the cases of fabrications by electron beams orfocused ion beams, the three-dimensional fabrication technique presentedabove does not require any reactive gas to be directed to thefabrication surface. Therefore, superior uniformity and precision offabrication can be achieved. When the target object 91 is Si, a FABcomprised of gaseous particles such as Cl₂, or SF₆ or CF₄ may be used.When the target object 91 is GaAs, a FAB comprised of chlorine gas maybe used.

In any of the methods presented above, the target object material is notlimited to those used for semiconductor production such as Si, SiO₂ orGaAs, and ceramics, glass, resins and polymers can be fabricated equallyeffectively. In these materials also, fine patterns can be produced withhigh precision, because there is no danger of accumulating electricalcharges or degradation in linearity of the beam caused by the electricalcharges. The target object material may also be a functionary gradientmaterial comprising at least two components of metals, semiconductorsand insulator in various composite forms. The combination may be any ofmetals and semiconductors, metals and insulators, semiconductors andinsulators, and metals and semiconductors and insulators. In thesematerials also, fine patterns can be produced over a large area withoutbeing affected by charge accumulation and loss of linearity.

As explained above, according to the method of the present invention,ultra-fine fabrication is possible with a precision range of 0.1 to 10nm, 10 to 100 nm, either by radiating a fast atomic beam onto a targetobject having a fine-patterned photoresist film, or by radiating a fastatomic beam having an ultra-fine beam diameter onto the target objectwhile rotating and/or translating the target object according to somepattern. In contrast to fabrication by an ion or electron beam (which issusceptible to local or irregular changes in electrical potential whichmay exist on the fabrication surface, as well as having a problem ofdegraded linearity of the beam itself), a fast atomic beam, which is anelectrically neutral particle beam, is not affected by chargeaccumulation or by an electrical/magnetic field, and is able to producesuperior fabrication by maintaining linearity of the beam during thefabrication process. As a result of the superior linearity of the beam,the fast atomic beam is able to be radiated straight into ultra-finegrooves and holes to enable ultra-fine fabrication over a large area offabrication surface. Further, in contrast to an ion or electron beamwhich is unsuitable to large-area fabrication, the method is applicableto semiconductors, insulators, or any other types of materials over alarge area, without adversely affecting their electrical properties. Themethod is further applicable to precision fabrication of an ultra-finethree-dimensional pattern by radiating an ultra-fine beam of fast atomicparticles onto the fabrication surface and providing relative movementbetween the target object and the beam source according to the pattern.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. A method of ultra-fine fabrication of a surfaceof a target object comprising a semiconductor material composed of agroup III element and a group V element of the periodic table, saidmethod comprising:dispersing on said surface a plurality ofmicro-particles such that said micro-particles form shields over firstportions of said surface while remaining second portions of said surfaceremain unshielded; and radiating an energy beam toward said surface suchthat said energy beam etches said second portions thereof while saidmicro-particles shield said first portions thereof, thereby forming afabricated target object including a base having fine structuresprotruding therefrom.
 2. A method as claimed in claim 1, wherein saiddispersing comprises applying to said surface a solution containing adispersion of said particles and a solvent.
 3. A method as claimed inclaim 2, wherein said applying comprises immersing said target object insaid solution.
 4. A method as claimed in claim 3, wherein saiddispersing further comprises removing said target object from saidsolution and evaporating said solvent from solution remaining on saidsurface, such that said micro-particles remain on said surface.
 5. Amethod as claimed in claim 2, wherein said applying comprises drippingsaid solution onto said surface.
 6. A method as claimed in claim 5,wherein said dispersing further comprises evaporating said solvent fromsaid solution on said surface, such that said micro-particles remain onsaid surface.
 7. A method as claimed in claim 1, wherein saidmicro-particles have a range of particle size of 0.1-10 nm.
 8. A methodas claimed in claim 1, wherein said micro-particles have a range ofparticle size of 10-100 nm.
 9. A method as claimed in claim 1, whereinsaid micro-particles have a range of particle size of 100 nm-10 μm. 10.A method as claimed in claim 1, wherein said micro-particles have arange of particle size of 0.1-10 nm and are selected from the groupconsisting of ferrite particles, zinc particles, cobalt particles anddiamond particles.
 11. A method as claimed in claim 1, wherein saidmicro-particles have a range of particle size of 100 nm-10 μm, and areselected from the group consisting of aluminum particles, graphiteparticles, gold particles and silver particles.
 12. A method as claimedin claim 1, comprising radiating said energy beam toward said surface ina direction substantially perpendicular thereto.
 13. A method as claimedin claim 1, wherein said energy beam comprises a beam selected from thegroup consisting of a fast atomic beam, an ion beam, an electron beam, aradiation beam, an atomic beam and a molecular beam.
 14. A method asclaimed in claim 1, wherein said energy beam etches said second portionswhile substantially not etching said micro-particles, such that saidfine structures are formed to have a rod shape.
 15. A method as claimedin claim 1, wherein, as said energy beam etches said second portions,said energy beam also progressively etches said micro-particles, suchthat said fine structures are formed to have a cone shape.
 16. A methodclaimed in claim 1, further comprising exposing said target object togaseous particles that are reactive with said target object, therebyperforming isotropical etching of said fine structures.
 17. A method asclaimed in claim 16, further comprising controlling the temperature ofsaid target object during exposure to said gaseous particles, andthereby controlling the chemical reaction of said gaseous particles onsaid target object and controlling said isotropical etching.
 18. Amethod as claimed in claim 1, wherein said dispersing and positioningcomprises locating separate said microparticles at spaced, separatelocations on said surface.
 19. A method as claimed in claim 1, whereinsaid fine structures have a property different from a correspondingproperty of said base.
 20. A method of ultra-fine fabrication of asurface of a target object comprising a semiconductor material selectedfrom at least one of a group consisting of GaAs, InAs, AlGaAs andInGaAs, said method comprising:dispersing on said surface a plurality ofmicro-particles such that said micro-particles form shields over firstportions of said surface while remaining second portions of said surfaceremain unshielded; and radiating an energy beam toward said surface suchthat said energy beam etches said second portions thereof while saidmicro-particles shield said first portions thereof, thereby forming afabricated target object including a base having fine structuresprotruding therefrom.
 21. A method of ultra-fine fabrication of asurface of a target object, said method comprising:dispersing on saidsurface a plurality of micro-particles such that said micro-particlesform shields over first portions of said surface while remaining secondportions of said surface remain unshielded; generating a fast atomicbeam having a large diameter from a beam source having flat plateelectrodes; and radiating said beam toward said surface such that saidbeam etches said second portions thereof while said micro-particlesshield said first portions thereof, thereby forming a fabricated targetobject including a base having fine structures protruding therefrom. 22.A method as claimed in claim 21, wherein said dispersing comprisesapplying to said surface a solution containing a dispersion of saidparticles and a solvent.
 23. A method as claimed in claim 22, whereinsaid applying comprises immersing said target object in said solution.24. A method as claimed in claim 23, wherein said dispersing furthercomprises removing said target object from said solution and evaporatingsaid solvent from solution remaining on said surface, such that saidmicro-particles remain on said surface.
 25. A method as claimed in claim22, wherein said applying comprises dripping said solution onto saidsurface.
 26. A method as claimed in claim 25, wherein said dispersingfurther comprises evaporating said solvent from said solution on saidsurface, such that said micro-particles remain on said surface.
 27. Amethod as claimed in claim 21, wherein said micro-particles have a rangeof particle size of 0.1-10 nm.
 28. A method as claimed in claim 21,wherein said micro-particles have a range of particle size of 10-100 nm.29. A method as claimed in claim 21, wherein said micro-particles have arange of particle size of 100 nm-10 μm.
 30. A method as claimed in claim21, wherein said micro-particles have a range of particle size of 0.1-10nm and are selected from the group consisting of ferrite particles, zincparticles, cobalt particles and diamond particles.
 31. A method asclaimed in claim 21, wherein said micro-particles have a range ofparticle size of 100 nm-10 μm, and are selected from the groupconsisting of aluminum particles, graphite particles, gold particles andsilver particles.
 32. A method as claimed in claim 21, comprisingradiating said energy beam toward said surface in a directionsubstantially perpendicular thereto.
 33. A method as claimed in claim21, wherein said energy beam etches said second portions whilesubstantially not etching said micro-particles, such that said finestructures are formed to have a rod shape.
 34. A method as claimed inclaim 21, wherein, as said energy beam etches said second portions, saidenergy beam also progressively etches said micro-particles, such thatsaid fine structures are formed to have a cone shape.
 35. A method asclaimed in claim 21, wherein said target object comprises asemiconductor material composed of a group III element and a group Velement of the periodic table.
 36. A method as claimed in claim 21,wherein said target object comprises a semiconductor material selectedfrom at least one of a group consisting of GaAs, InAs, AlGaAs andInGaAs.
 37. A method as claimed in claim 21, wherein said target objectcomprises a semiconductor material selected from the group consisting ofSi and SiO₂.
 38. A method claimed in claim 21, further comprisingexposing said target object to gaseous particles that are reactive withsaid target object, thereby performing isotropical etching of said finestructures.
 39. A method as claimed in claim 38, further comprisingcontrolling the temperature of said target object during exposure tosaid gaseous particles, and thereby controlling the chemical reaction ofsaid gaseous particles on said target object and controlling saidisotropical etching.
 40. A method as claimed in claim 21, wherein saiddispersing and positioning comprises locating separate saidmicro-particles at spaced, separate locations on said surface.
 41. Amethod as claimed in claim 21, wherein said fine structures have aproperty different from a corresponding property of said base.